Joe,
Sounds to me like you have BGA's that have warped during the reflow process
- probably due to excessive moisture content. Either that or your PCB is
warped. Do you monitor and control moisture exposure levels for you
components, or do you bake them out prior to mounting?
Two other factors that I've experienced, and which may contribute to your
problem, are those of solder ball diameter & "form", and of solder paste
deposition height/volume. I have had BGA's where the unmounted volume and
shape of the balls have varied dramatically - I even had one once that had
a bond wire included in a ball. Solder volume in the finished joints should
vary only very marginally, or the joint characteristics become to disparate
to be reliable long-term. However, solder joint volume, unless you have
gross variance, should not really give you a variation in stand-off height
unless the surfaces that you're measuring between are not flat. Back to my
initial diagnosis. Do your solder joints vary from long and thin to short
and fat?
Peter
"Macko, Joe @ IEC" <Joe.Macko 10/12/2002 06:57 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to "Macko, Joe @
IEC"
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] BGA standoff distance
Fellow Technet subscribers,
I have a question related to BGA standoff distance after reflow. We are
noticing some variation in standoff distance on some of the larger plastic
BGAs installed onto densely populated, double sided boards. By standoff
distance I mean the distance measured between the bottom of the BGA package
and the top of the solder mask located on the pwb.
Is standoff distance considered a good indicator of correct solder ball
collapse? Is there a formula for calculating what the theoretical standoff
distance should be after reflow based on package weight, solder ball
diameter, # of balls, etc...?
Joe
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
-----------------------------------------------------
[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
person. Thank you.]
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
